There have conventionally been known techniques for imparting flame retardancy to synthetic resins such as styrene resins and polyester resins. For example, JP-A-50-27843 (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) discloses an expedient of incorporating a halogenated bisphenol A epoxy resin, as a flame retardant, having low volatility and excellent non-bleeding properties, heat resistance, and other properties and, according to need, further incorporating antimony trioxide or the like as a flame retardant aid. JP-A-62-4737 discloses an expedient of modifying an epoxy resin for use as a flame retardant by blocking the epoxy groups in the epoxy resin with a halogenated phenol such as tribromophenol.
However, the incorporation of a halogenated bisphenol A epoxy resin as disclosed in JP-A-50-27843 has had a problem that the thermal stability of the synthetic resin whose flame retardancy should be improved is significantly impaired by the epoxy resin. Illustratively stated, when the epoxy resin-incorporated synthetic resin composition is kneaded in an extruder and molded with an injection molding machine, epoxy groups in the epoxy resin undergo a polymerization reaction according to the heat hysteresis applied to the molten resin composition and, as a result, the epoxy resin gels, causing the composition to show poor flowability during molding, suffer discoloration, and come to contain aggregates resulting from gelation.
On the other hand, the flame retardant disclosed in JP-A-62-4737 has had a defect that although the proposed epoxy resin has eliminated the gelation problem, it impairs the light resistance of synthetic resins more than unmodified halogenated bisphenol A epoxy resins in which the epoxy groups are unblocked.
Since light resistance is an important requirement particularly for styrene resins in view of their actual uses, there has been an intense desire to solve the problem that flame-retardant synthetic resins excellent in both thermal stability and light resistance have been unable to be obtained.